I purchased a 1960s Magnavox console recently, and have fallen in love with the amp's warm sound (though I can't figure out what model number it is--see attached picture). However, it has begun to hum quite a bit. I replaced all the tubes (4 6V6, 2 6EU7, 1 6CA4), and checked and cleaned all connections--no reduction of hum. I think I need to recap the whole thing since it's all original parts, but I know absolutely nothing about capacitors. For starters, I think I need to replace the power supply capacitor--the big aluminum-can looking thing on top of the chassis, right? I can't figure out where one buys such a thing, however--I've looked at DigiKey but don't really understand all the options. Is there an online starter's guide or a book that would be useful for a complete newbie to DIY tube amp repair? Any help for an ignorant but interested DIY-er is greatly appreciated!

The likely cause of hum would be a failing power supply capacitor or a failing tube. For replacement power supply capacitors try Antique Electronic Supply at:

http://www.tubesandmore.com

Good luck. With the right speakers those Magnavox amplifiers can sound unbelievably good.

Thanks for the guidance, tcdriver--I pulled out the filter capacitor, and it shows signs of leakage and corrosion, so I assume that's the problem. Another question for anybody out there--if I replace the multi-stage filter capacitor, how high can the new values be? The old capacitor is a four-stage can, FP-type, with these values: 40uF@350VDC, 40uF@350VDC, 20uF@300VDC, 30uF@25VDC. Do I need to have the exact same capacitor (which I'd have to get custom-built), or can I replace it with something that's, say, 40/40/30/30 mfd @ 475vdc?

Thanks for the help!

You can replace it with separate caps on a terminal strip - they're small enough to fit under the chassis. If you want, put the can back to fill up the hole...

I think yours says model AMP8502 - not listed in Sams Photofact index, unfortunately. If I misread it and it's 8802, it's in #591-8. I have a bunch of schematics, but all are earlier than this one.

Do I need to have the exact same capacitor (which I'd have to get custom-built), or can I replace it with something that's, say, 40/40/30/30 mfd @ 475vdc?

Thanks for the help!
The 40/40/30/30 mfd @ 475vdc should work fine. However, you may do better by using the first two sections just as the amplifier is currently configured then parallel the two 30uF sections and then use a separate 30uF@25VDC to replace that last section.

Okay, so I've decided to go the route of putting new caps on a terminal strip under the chassis; I'm not going to worry about the can since I never see the innards of this thing except when I'm trying to fix it. I'm confused, however, about how I would go about putting the final stage--the 30uF@25VDC--in "separately."

By the way, the model no. of the amp is 8802-00 (same as 8801-00), which I have a schematic for from Sams Photofact, should anyone be interested.

By the way, I might also mention that the attempt to learn about tube amp repair is especially thrilling for a professor of history who, as of two days ago, knew absolutely zip about electronics. I greatly appreciate everybody's help in this endeavor!

Just solder the terminal strips below the chassis, and solder the wires to them that were soldered to the pins on the old can. The new ones will be marked (+) and (-). (-) goes to the chasssis, (+) to the wire that came from the can pin. This is assuming that the can was grounded to the chassis.

What you probably want are "axial electrolytics" . Sprague "Atom" capacitors are one type that probably come in most of the values you need.

Any working voltage will do as long as it is the same or higher than the original. It's also ok to go a little higher on capacitance than original if you want or if you cant find the right value. Just don't go lower. I would put the old can back on for looks.

John

Okay, so I've decided to go the route of putting new caps on a terminal strip under the chassis; I'm not going to worry about the can since I never see the innards of this thing except when I'm trying to fix it. I'm confused, however, about how I would go about putting the final stage--the 30uF@25VDC--in "separately."
The 30uF@25VDC capacitor is the cathode bypass capacitor. The Magnavox amplifiers commonly used a single cathode resistor to bias all four output tubes and then used a single bypass capacitor across the cathode resistor. You may confirm this is the case with your amplifier by checking to see that the positive side of the original 30uF@25VDC capacitor was connected to the cathodes of the output tubes. The can casing is connected to the chassis ground and you should confirm that the common cathode resistor is also connected to the chassis ground.

One easy way to mount it would be to solder the negative end to the ground side of the common cathode resistor and solder the positive end to the side of the cathode resistor tied to the tube cathodes. CAUTION, only do this if you can maintain at least one inch separation between the body of the cathode resistor and the bypass capacitor. The cathode resistor gets hot and capacitors do not last long if they get too hot.

Thanks, guys. This has been so helpful!

2000,
Don't overlook any diodes in the pwer supply. Another possible cause of hum.
BTW. Welcome to AK!
Ron

To wrap up this thread:
I replaced the can capacitor in the power amp with four separate Sprague Atom caps, attached to a terminal strip under the chassis. I also replaced all paper caps with Orange Drops.

I found this website especially helpful: http://antiqueradio.org/recap.htm

Of course, once I hooked up the recapped power amp I discovered it was actually the preamp causing the hum (doh!). So I replaced the can capacitor there, using the same method. Voila! The whole unit sounds so fantastic I can hardly believe it--no hum, crystal clear highs, warm mids, and huge bass.

Thanks for the help, all.

Thanks for the update. I am glad to hear that you got your Magnavox sounding the way it should. Even though the problem as it turned out was in the preamplifier, the updating of the power amplifier capacitors is a worthwhile exercise. Good listening.